In recent years, there have been actively made research and development of not only industrial robots but also consumer robots, which play various roles for people's livelihood. Among such robots, human-like robots (humanoid robot) being able to walk in erect posture are expected to be able to substitute for actions of human beings. In such humanoid robots, many joint parts are provided for imitating actions or motions of a human being, and a variety of motions having a plurality of degrees of freedom in the joint parts are required.
For example, in FIG. 6 of patent literature 1, there is disclosed a joint structure of a leg unit of a robot. Specifically, a thigh part located at an upper side of the leg unit and a crus part located at a lower side of the leg unit are connected with a plate member so as to be rotatable relative thereto. Then, these parts and member are arranged in such a manner that when the leg unit becomes the most extended state, the plate member abuts against a stopper at the side of the thigh part and a stopper at the side of the crus part. With such an arrangement, when the robot extends its leg units and stands erect, each stopper and the plate member become an abutted state due to the self weight of the robot, so it becomes possible to maintain the erect state of the robot in a relatively stable manner. In addition, in order to maintain the contact state of the stoppers and the plate member at the time of the erect standing of the robot, an arrangement is also adopted in which springs are provided at the thigh part side and at the crus part side, respectively, to apply resilient forces.